The use of supercritical fluids as a transport medium for the manufacture of surface coatings is well known. For example, U.S. Pat. No. 4,923,720 discloses the use of supercritical fluids to reduce the consistency of viscous coating compositions to allow for their application as liquid sprays.
A particular type of coating commonly used in industrial applications is a mold release agent. Mold release agents find use in such varied arts as molding plastics, resins and elastomers, baking food products and coating surfaces, all of which utilize a release surface to prevent the adhesion of plastics, resins, food products and coatings, as the case may be, to the substrate to which they are supplied or applied.
One common method of applying a mold release agent to a substrate is to spray the agent onto the mold. The use of a supercritical fluid as a viscosity reduction medium in mold release spray applications has become more common with the environmental hazards associated with solvents and other volatile organic compounds.
U.S. Pat. No. 4,882,107 discloses a mold release coating process and apparatus using supercritical fluids. In either batch or continuous processes, predetermined amounts of supercritical fluid and mold release agent are both measured prior to mixing and being sprayed onto a substrate.
Measurement of the supercritical fluid flowrate may be accomplished by a variety of methods as the physical properties of the fluid are predictable. However measurement of the mold release agent is more complicated. Mold release agents are commonly waxes suspended in a solvent. The mixture has a viscosity which varies widely with shear rate, which increases the difficulty of flow measurement. The wax suspended in the solvent also tends to settle out or agglomerate and clog passageways in typical measurement metering devices, thus rending these devices unreliable.
Therefore it is of particular interest in the art to supply a controlled amount of a mold release agent to be mixed with a supercritical fluid which does not require the use of multiple flow meters to measure both the supercritical and non-compressible fluid.